The present invention is generally related to both paper machines and paper finishing machines, such as calenders and coating devices, used in manufacture of paper. In these machines, various press nips are employed, in the press section of the paper machine press nips that remove water and/or equalize the web, in calenders calendering nips, and in coating devices coating nips, in which the coating agent is transferred onto the web.
The scope of the invention also includes so-called extended-nip presses, at which the hose roll face or equivalent that runs through the press zone, which press zone is longer than in a normal roll nip, and/or the transfer belt face which runs through the extended-nip zone and which transfers the web further is/are monitored.
The faces of said rolls and the faces of felts and bands running through the nips are susceptible of contamination and damage. As is known from the prior art, said faces are conditioned and cleaned, and, in some cases, their cleanliness is also monitored by means of particular methods and apparatuses.
A particularly favourable embodiment of the invention concerns monitoring of the condition of calender rolls, in particular of soft-coated calender rolls, in order to detect any damage in their faces at a sufficiently early stage and, thus, to permit prevention of damage in advance.
As is known from the prior art, calenders comprise two or more hard-faced and/or soft-faced calender rolls, which form a calendering nip or nips with each other, through which nip/nips the paper web to be treated is passed. In particular rolls with soft faces, such as paper rolls or equivalent in supercalenders and rolls provided with soft coatings, in particular polymer coatings, in what is called soft calenders, are susceptible of damage. The reason for the damage is frequently contaminations, such as local fiber strings, which cause a pressure impact when they pass through the nip, which impact loads the soft coating on the calender roll and first causes its heating and, in the long run, a permanent deformation in the coating and damage. Similar deformations and damage also occur in metallic roll faces and in the faces of bands or felts running through nips, as well as in the face of the flexible mantle of a hose roll and/or of a transfer band running through so-called extended-nip zones.
As is known from the prior art, for monitoring the condition of calender rolls, in particular of soft-faced rolls, thermometers traversing in the cross direction of the machine have been used, by whose means the temperature of the coating is monitored. With respect to this technique, reference is made to the U.S. Pat. No. 4,642,164, the monitoring of the temperature described in said patent being a part of the gloss regulation system of a supercalender. In the temperature monitoring application in accordance with said U.S. patent, and also in corresponding other prior-art systems, problems arise from the fact that the resilient roll coating, whose temperature is monitored, is, as a rule, at least to some extent electrically insulating. Thus, in the partly rubbing contact between the web and the coating, quite high charges of static electricity arise in the faces of the roll coating and of the relatively dry paper web. These charges tend to be discharged along the available routes with the lowest resistance. A thermographic camera must often be installed so that it projects from its support construction, in which case said static electric charges find exactly the thermographic camera as the easiest route of discharge, in which connection the sensitive electronic system of the thermographic camera is subjected to quite high voltages, and it must be protected specifically against such voltages.
Even if monitoring of the temperature of the face of a calender roll usually permits detecting of a local raised temperature resulting from an excessive load applied to the coating or from a local inner non-homogeneity at a sufficiently early stage, this requires installation of quite heavy, expensive and spacious equipment in the vicinity of the roll to be monitored. In particular, congestion of space causes great difficulties in connection with multi-roll treatment devices, at which every device that is not included in the web treatment process proper makes the maintenance and servicing of the device more difficult.
Besides calenders, the environments of application of the present invention also include other finishing devices, such as coating devices and various nips in a paper machine, including press nips that are provided with a felt or felts and/or with a transfer belt and that remove water and/or equalize the web, the following being stated in respect of the prior art related to said press nips.
As is well known, the presses in a paper machine comprise press rolls, which form press nips with each other, through which nips the paper web runs together with a press felt or between press felts. Press rolls with their axle journals and bearing supports are susceptible of vibration, which is increased by the elasticity of the press felts. In press rolls, self-vibration commonly occurs, which is typically in the frequency range of about 20 to about 150. The vibrations cause wave-shaped barring in the press felts and extra wear of the felts. Said barring is adapted to increase the so-called felt-induced vibration of press rolls, whose amplitudes tend to be increased constantly as the wave-shaped barring in the felts becomes deeper.
Attempts have been made to prevent the felt-induced vibration in press rolls thereby that the direction of progress of the felt is varied at suitable intervals by a few angular degrees so that the felt runs over spreader, tensioning and alignment rolls in slightly varying positions. Said varying of the direction of progress means that the felt loop runs in a slightly oblique position, i.e. the length of one of its edges is slightly larger than the length of the opposite edge.
In respect of the prior art related to the press applications of the present invention, reference is made, by way of example, to the paper in Tappi Journal, July 1987, pp. 49 to 54, by Yvon Phil Gagnon, "Low-frequency vibration analysis of paper machine presses using displacement transducers and the synchronous time-averaging method".
Further, with respect to press section applications of the present invention, reference is made to the current assignee's U.S. Pat. No. 5,403,447. In said patent, a system is described in the press section of a paper machine for monitoring and controlling the running of the press felts, which press felts are controlled by means of rolls, whose axial direction has been arranged to be varied by means of an actuator for the purpose of controlling the running of the press felts. This system includes detector devices, by whose means the alignment of the felt or felts is detected, as well as vibration detectors, by whose means vibrations of the press rolls and/or of related parts are detected. The system includes a computer or an equivalent logic unit, to which the monitoring signals are fed from said detectors. The computer or equivalent has been programmed to analyze detector data in particular in order to detect felt-induced vibrations. By means of the control system connected to it, the computer regulates the actuators of the guide rolls that control the running of the felts so that, when the vibration level of the press rolls rises beyond preset limits, the alignment of the guide roll or guide rolls of the felt that causes the vibration is altered until an acceptable level of vibration and/or a level of vibration as low as possible is found.
By means of the arrangements in accordance with the prior art coming out from the above cited papers, it is possible to detect vibrations arising from asymmetry of the construction of the press rolls only, and by means of the system U.S. Pat. No. 5,403,447 it is also possible to locate the press felt that is the reason for the vibrations. By means of said arrangements, it is, however, not possible to monitor the impurities and structural damage in the faces of the felts running over the rolls and through the press nips, and, by means of the prior-art arrangements, it is not possible to control the conditioning operations so that they are applied efficiently exactly in the area of the impurities.